Epilepsy is a collection of disorders that causes recurrent seizures. Often it is not the fault of a single gene but of complex genetic interactions. Each year, 200,000 new cases of epilepsy are diagnosed. In seventy percent of these cases there is no apparent cause. It is estimated that ten percent of these new cases will never obtain adequate seizure control even with treatment. Currently, the major focus of research is on improving and devising anticonvulsant medications, but the underlying mechanisms need to be uncovered and better understood to improve treatment strategies. To date, most of the known epilepsy genes are monogenic and responsible for human autosomal dominant forms of the disease. The more complex epilepsies are likely the result of polygenic interactions and environment. Our long-term goal is to elucidate the mechanisms by which a mutation in the Dynamin-1 (Dnm1) gene causes a complex seizure disorder. Dnm1 encodes a large multimeric GTPase necessary for activity-dependent membrane recycling in neurons, including synaptic vesicle endocytosis. Mice heterozygous for a novel spontaneous Dnm1 mutation - fitful - experience recurrent seizures, and homozygotes have more debilitating, often lethal seizures in addition to severe ataxia and neurosensory deficits. Fitful is a missense mutation in an exon that defines the Dnm1a isoform, leaving intact the alternatively spliced exon that encodes Dnm1b. We hypothesize that in fitful mice endocytosis is stalled at a checkpoint that requires the proper function of the Dnm1 isoforms to proceed from early endocytic events to later stage fission events. A delay in synaptic vesicle endocytosis would result in a deficiency of readily available vesicles for neurotransmission. Using fitful as a model of epilepsy, the following Specific Aims are proposed to address genetic questions related to the endocytic checkpoint: 1) Determine the genetic interactions involved in the endocytic checkpoint. We will use genetically disrupted mouse mutant strains of genes known to interact with Dnm1 at different stages of endocytosis to ask how disruption of endocytosis/fission at different points affects seizure phenotype differentially. 2) Dissect the potentially overlapping roles of the Dnm1 alternatively spliced isoforms in the checkpoint resulting in the seizure phenotype. Studies will utilize Dnm1 isoform specific mouse lines for animal genetic and seizure phenotype analysis. Collectively, these studies will define the involvement of Dnm1 in synaptic vesicle endocytosis. Additionally, they will give insight into the role that genes encoding endocytic proteins play in contributing to epilepsy. We suggest that using an in vivo model of dynamin dysfunction, such as fitful, will answer important questions not only about how dynamin functions in endocytosis, but also the important role dynamin plays in proper synaptic transmission.